Implement the local JS snippets RFC

This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:

* The `module` attribute disallows paths starting with `./` and `../`.
  It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
  rather emits an ES module that can be natively loaded into a browser.

Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

Closes #1311

crates/shared/src/lib.rs

@@ -14,16 +14,22 @@ macro_rules! shared_api {
             imports: Vec<Import<'a>>,
             structs: Vec<Struct<'a>>,
             typescript_custom_sections: Vec<&'a str>,
-            // version: &'a str,
-            // schema_version: &'a str,
+            local_modules: Vec<LocalModule<'a>>,
+            inline_js: Vec<&'a str>,
         }
 
         struct Import<'a> {
-            module: Option<&'a str>,
+            module: ImportModule<'a>,
             js_namespace: Option<&'a str>,
             kind: ImportKind<'a>,
         }
 
+        enum ImportModule<'a> {
+            None,
+            Named(&'a str),
+            Inline(u32),
+        }
+
         enum ImportKind<'a> {
             Function(ImportFunction<'a>),
             Static(ImportStatic<'a>),
@@ -113,6 +119,11 @@ macro_rules! shared_api {
             readonly: bool,
             comments: Vec<&'a str>,
         }
+
+        struct LocalModule<'a> {
+            identifier: &'a str,
+            contents: &'a str,
+        }
         }
     }; // end of mac case
 } // end of mac definition